Ecology

An evolutionary arms race

Amanda Markle
Strawberry Hill Nature Preserve

(6/2021) When considering the epic battles of nature, one's mind often turns to those fought by larger animals. Lions taking down an enormous elephant on the savannah. Seals who display astonishing feats of acrobatics as they dodge the ambushes of great white sharks. The kind of battles you would see in a nature documentary; a showing of claws and gnashing of teeth, accompanied by dramatic music and the harsh lesson that nature is not often kind to many of its inhabitants.

Yet not all battles play out in this made-for-tv fashion. Some of nature’s hardest-fought battles are also the most subtle. One such battle began millions upon millions of years ago; long before dinosaurs began squabbling amongst themselves, before teeth and claws even existed as we know them (and in fact, possibly even playing a role in the development of these flashier displays of dominance). A battle that rages on today, impacting everything from biodiversity to agricultural practices to medicine: the evolutionary battle between plants and herbivores.

Since as far back as we have a fossil record to study, plants and herbivores have been locked in a relentless struggle. Plants have always played a key role in the earth's ecosystems. Their ability to harness energy from the sun and convert it into a form usable by other organisms establishes them as the anchor for virtually all life on our planet, and a valuable commodity in nature. Almost as soon as plants came into existence, so did things that wanted to eat them.

Marine organisms were the first to develop herbivory, feeding on plants in ancient lakes and oceans. When the first terrestrial plants emerged 450 million years ago, animals on land quickly evolved to eat those, too. This constant cycle of adaptation and response as a method for survival is sometimes referred to as the Red Queen hypothesis. This references Lewis Carroll's Through the Looking Glass, and the Red Queen who explains to Alice: "Now, here, you see, it takes all the running you can do, to keep in the same place. If you want to get somewhere else, you must run at least twice as fast as that!"

In this battle for survival, herbivores play evolutionary offense. Insects, with their relatively short life cycles and large populations, can adapt quickly to take full advantage of the plants available in their ecosystem. In grasshopper populations, head size varies depending upon the food they are raised on- those raised on tougher grasses have larger heads, giving them extra jaw strength. Caterpillars within the same family will have different mandible structures based on whether they are tearing and chewing softer leaves, or cutting and eating tougher, more mature leaves. Larger herbivores too have evolved to have access to as much plant nutrition as possible. Elephant molars vary from species to species in both shape and crown size depending on the native plants available to them.

Without much mobility to escape would-be consumers, plants were almost immediately forced onto the defensive. Even as far back as 420 million years, the fossil record shows plants adorned with spiky protrusions; armor against the organisms that pierced and sucked nutrients from their stems. As plants on land evolved more complex structures, including roots and seeds, herbivores quickly evolved to eat them. In many populations, plant species must simply tolerate a certain level of herbivory. In places where water and light are readily available, plants may grow with enough abundance to outpace the needs of hungry herbivores, but where resources are scarce, plants tend to invest far more energy into protecting themselves. Structural defenses are mounted in the form of thorns and spines. Chemical warfare is waged, with plants producing substances that give off a bitter taste, inhibit herbivores' ability to digest, or poison those who would make a meal of them.

This evolutionary arms race has resulted in a stunning amount of biodiversity among plants and herbivores, particularly insects. With every new offensive strategy launched by the herbivores, plants were forced to adapt or be consumed. As the herbivores responded in kind, tiny changes added up to create new species in both combatants. Throughout history, explosions in the speciation of plants and insects have coordinated with one another as they coevolved, each trying to stay one step ahead of the other, leading to some highly specialized and truly fascinating adaptations.

Some plants are only able to store limited amounts of toxins to defend themselves and deploy them only to areas under attack. In response, insects will feed in groups so that toxins are spread thinly enough to not overwhelm any individual. Animals may ingest poisonous plants but follow their meal by eating clay or minerals to neutralize the toxins. Some animals, like the monarch butterfly, have evolved to store and use certain plant toxins to their advantage. Monarch larvae can build up cardiac glycosides from the milkweed on which they feed to make them unpalatable to their own predators. Other caterpillar species will physically manipulate their food sources, rolling mature leaves of a plant around the buds, blocking light from reaching the bud which prevents the development of undesirable tannins and keeps the bud tender and easy to eat.

Plant species have kept up with their herbivore foes with some impressive specializations of their own. Plants suitable for laying insect eggs and feeding developing larvae will develop spots that mimic the appearance of insect eggs, falsely signaling to a searching mother insect that its nursery is already full. Others have developed ways of recruiting mercenaries to come to their aid. Acacia trees provide both room and board, growing with hollow spikes ideally suited to house ants that keep away other insects and producing nectar that serves as a food source for the ants.

It is theorized that even the hallmarks of autumn may, on some level, be a defense against herbivores; leaves shifting from green to oranges, reds, and browns make it harder for many insects to camouflage themselves in trees and the shedding of leaves may prevent populations of leaf-mining insects from reaching levels that would overwhelm the trees. While plants are typically on the defensive end of coevolution with herbivores, they occasionally do take on an offensive strategy.

Some plants that have taken a more aggressive approach to their interactions with herbivores are well known, like Venus flytraps and pitcher plants, but scientists estimate there are more than 500 species of carnivorous plants. Even your backyard tomato plants have a predatory side, trapping tiny aphids on their sticky stems that eventually drop down and decay into fertilizer for the growing plant.

Aside from the spectacular amount of biodiversity sparked by this battle between plants and herbivores, the constant competition has resulted in several adaptations that humans have been able to use to their benefit. Many pharmaceuticals, including opium, quinine, and aspirin are derived from plant compounds that evolved as herbivore deterrents. Selecting crop species with natural hardiness against pests allows farmers to drastically decrease the pesticides they use. Even the human practice of cooking food is thought to have developed in part to break down harmful plant toxins and make plant nutrition more readily available. Adaptations and responses between plants and herbivores are one of the most concrete ways humans can observe evolution in action; an epic battle spanning millions of years, with strategies that continue to change and surprise even to this day!

Amanda Markle is the Environmental Education Manager of the Strawberry Hill Foundation. Strawberry Hill inspires stewardship of our natural world by
connecting the community with educational opportunities.
 Learn more by visiting StrawberryHill.org.